Amino acid incorporation was measured in a cell-free protein synthesis system derived from the brains of male Fischer-344 rats of different ages. This system has the capacity to initiate protein synthesis in vitro. There was no significant correlation between protein synthesis and age. Analysis of neurofilament proteins in the rat central nervous system by two dimensional gel electrophoresis revealed the three neurofilament proteins of 210, 160, and 68K daltons. Heterogeneity existed in the protein pattern, especially in the 68K daltons polypeptide. Analysis of gene expression in 11 individual regions of the rat brain at the level of protein end-product was conducted using two dimensional gel electrophoresis in conjunction with a sensitive silver stain protein detection procedure. Differential gene expression was observed for a number of brain proteins, but the level of most brain proteins did not change with age. The levels of mRNA associated with the translational apparatus in 3 regions of the rat brain was studied using two dimensional gel electrophoretic analysis of the radiolabeled products of in vitro translation of brain free and membrane bound polyribosomes. Differential gene expression was observed for a number of brain messenger RNAs. The majority of brain messenger RNAs in the three brain regions did not change with age. Levels of the cellular homologue of the Rous sarcoma virus, c-src, were examined in rat brain as a function of age, using Northern blotting and nucleic acid hybridization and immunoprecipitation of translation products. The levels of expression of c-src are age invariant in the rat brain.